1. Field of the Invention
This invention relates generally to a method for preparing one or more specific dimethylnaphthalene isomers by dehydrogenating one or more specific dimethyltetralin isomers.
2. Description of the Prior Art
Naphthalene dicarboxylic acids are monomers that are known to be useful for the preparation of a variety of polymers. For example, poly(ethylene 2,6-naphthalate) prepared from 2,6-naphthalene dicarboxylic acid and ethylene glycol has better heat resistance and mechanical properties than polyethylene terephthalate and is useful in the manufacture of films and fibers.
Dimethylnaphthalenes are desirable feedstocks for oxidation to the corresponding naphthalene dicarboxylic acids. A known conventional process for producing a naphthalene dicarboxylic acid comprises the oxidation of a dimethylnaphthalene with oxygen in the liquid phase in an acetic acid solvent at an elevated temperature and pressure and in the presence of a catalyst comprising cobalt, manganese and bromine components.
Typically dimethylnaphthalenes are found in refinery or coal-derived streams as mixtures of all of the ten possible dimethylnaphthalene isomers. However, separation of these isomers is very difficult and expensive. Consequently, methods for producing specific dimethylnaphthalenes or mixtures of two or three specific dimethylnaphthalenes in high purity and quality are highly desirable. One type of such method is a multistep synthesis involving (1) the formation of an alkenylbenzene by the reaction of o-, m- or p-xylene or ethylbenzene with butadiene, (2) the cyclization of the resulting alkenylbenzene to form one or more dimethyltetralins belonging to one or two of three groups of three isomeric dimethyltetralins --that is, either group A containing the 1,5-, 1,6-, 2,5- and 2,6-dimethyltetralins, group B containing the 1,7-, 1,8-, 2,7- and 2,8-dimethyltetralins, or group C containing the 1,3-, 1,4-, 2,3-, 5,7-, 5,8- and 6,7-dimethyltetralins--(3) the dehydrogenation of the dimethyltetralin(s) to form the corresponding dimethylnaphthalene(s), and (4) the isomerization of the resulting dimethylnaphthalene(s) to the desired specific dimethylnaphthalene.
For example, Thompson, U.S. Pat. Nos. 3,775,496; 3,775,497; 3,775,498; 3,775,500 disclose processes for the cyclization of specific alkenylbenzenes to one or more specific dimethyltetralins at 200.degree.-450.degree. C. in the presence of any suitable solid acidic cyclization catalyst such as acidic crystalline zeolites as well as silica-alumina, silica-magnesia and silica-alumina-zirconia and phosphoric acid, followed by the dehydrogenation of the resulting dimethyltetralin(s) in the vapor state to the corresponding dimethylnaphthalene(s) in a hydrogen atmosphere at 300.degree.-500.degree. C. and in the presence of a solid dehydrogenation catalyst such as noble metals on carriers and chromia-alumina, and thereafter isomerization of each of the aforesaid dimethylnaphthalene(s) to the desired isomer within the triad of dimethylnaphthalenes to which the isomer being isomerized belongs, at 275.degree.-500.degree. C. in the presence of a solid acidic isomerization catalyst of the same type as described in respect of the cyclization disclosed therein. In the alternative, both the cyclization and isomerization reactions can be performed in the liquid phase, in which case the cyclization is performed at 200.degree.-275.degree. C. with a solid phosphoric acid catalyst, at 70.degree.-140.degree. C. with an acidic ion exchange resin, an acidic crystalline zeolite, hydrofluoric or sulfuric acid as the catalyst or a siliceous cracking catalyst.
More specifically, Thompson, U.S. Pat. No. 3,775,496 discloses the cyclization of 5-(m-tolyl) -pent-2-ene to 1,6- and 1,8-dimethyltetralins, which are then dehydrogenated to 1,6- and 1,8-dimethylnaphthalenes, which in turn are isomerized to 2,6- and 2,7-dimethylnaphthalenes, respectively. Thompson, U.S. Pat. No. 3,775,497 discloses the cyclization of 5-phenyl-hex-2-ene to 1,4-dimethyltetralin which is then dehydrogenated to 1,4-dimethylnaphthalene, which is in turn isomerized to 2,3-dimethylnaphthalene. Thompson, U.S. Pat. No. 3,775,498 discloses the cyclization of 5-(o-tolyl)-pent-2-ene to 1,5-dimethyltetralin, which is then dehydrogenated to 1,5-dimethylnaphthalene, which is in turn isomerized to 2,6-dimethylnaphthalene. Thompson, U.S. Pat. No. 3,775,500 discloses the cyclization of 5-(p-tolyl)-pent-2-ene to 1,7-dimethyltetralin, which is then dehydrogenated to 1,7-dimethylnaphthalene, which in turn is isomerized to 2,7-dimethylnaphthalene.
Shimada et al., U.S. Pat. No. 3,780,119 disclose a method for the isomerization of dimethylnaphthalene by the use at a temperature above 260.degree. C. of a mordenite catalyst in which a metal form of mordenite is in excess of 20 weight percent of the mordenite, with the metal being selected from the group consisting of lithium, sodium, potassium, magnesium, calcium, strontium, barium, zinc and aluminum.
Suld et al., U.S. Pat. No. 3,803,253 disclose a method for the hydroisomerization of a dimethylnaphthalene by the use of a combination of a hydrogenation catalyst and a calcium-containing zeolite catalyst, such as a calcium-exchanged synthetic faujasite, for example, a Y-type molecular sieve.
Shima et al., U.S. Pat. No. 3,806,552 disclose a method for the isomerization of dimethylnaphthalenes in the gas phase by the use of a mixed catalyst consisting of (a) 65-95 weight percent of a hydrogen form of mordenite in which above 80 weight percent of the metal cations are replaced with hydrogen ions, and (b) 5-35 weight percent of catalyst selected from the group consisting of bentonite and fuller's earth.
Hedge, U.S. Pat. No. 3,855,328 discloses a method for the isomerization of dimethylnaphthalenes by the use of a Type Y alumino silicate zeolite at 120.degree.-300.degree. C. in the liquid phase. The catalysts have aluminum-to-silicon atomic ratios of 0.1-1.0.
Ogasawara et al., U.S. Pat. No. 3,888,938 disclose a method for the isomerization of dimethylnaphthalenes in the liquid phase by the use of a mixed catalyst consisting of (a) 70-95 weight percent of a hydrogen form of mordenite in which above 80 weight percent of the metal cations are replaced with hydrogen ions, and (b) 5-30 weight percent of a promoter selected from the group consisting of bentonite and fuller's earth.
Hedge et al., U.S. Pat. No. 3,928,482 disclose the isomerization of either dimethyldecalins, dimethyltetralins or dimethylnaphthalenes in the presence of an alumino silicate zeolite containing polyvalent metal cations in exchange positions, such as a rare earth-exchanged Type Y zeolite.
Yokayama et al., U.S. Pat. No. 3,957,896 disclose the selective isomerization of dimethylnaphthalenes in the presence of any kind of natural or synthetic, solid acid catalyst, such as Y-type zeolite as well as silica-alumina, silica-magnesia, silica-zirconia, silica-aluminazirconia, fuller's earth, natural or synthetic mordenite, X-type zeolite, A-type zeolite and L-type zeolite. These catalysts may be substituted partly or wholly by hydrogen or metal. Furthermore, these catalysts can be unsupported or supported on carriers.
Onodera et al., U.S. Pat. No. 4,524,055 discloses a crystalline aluminosilicate zeolite that is useful in the isomerization of dimethylnaphthalenes and has a silica-to-alumina mole ratio of 10 to 100, specific x-ray lattice distances, and a specific cylohexane-to-n-hexane adsorption ratio of at least 0.7.
Maki et al., U.S. Pat. No. 4,556,751 disclose the isomerization of dimethylnaphthalenes in the presence of a crystalline aluminosilicate having a pentasil structure and a silica-to-alumina molar structure of 12 or higher. In addition, the aluminosilicate may contain some other metals as non-exchangeable metals.
A problem in all such prior art methods is the presence of other dimethylnaphthalene isomers and unconverted dimethyltetralin as impurities and by-products in the finally obtained, desired specific dimethylnaphthalene isomer. The presence of such impurities and by-products markedly reduces the utility and commercial value of the desired dimethylnaphthalene isomer, especially as a precursor for the formation of a naphthalene dicarboxylic acid for use as a monomer in the manufacture of a polymer. In addition, catalysts tend to deactivate relatively rapidly at the high temperatures employed in vapor phase processes. Therefore, it is highly desirable to employ liquid phase processes and to improve the completeness of each step in the aforesaid multistep synthesis and the selectivity of each step therein for the production of the desired product therefrom.
In this regard, it is known that in the presence of an acid catalyst, the dimethylnaphthalene isomers are isomerizable within each triad of dimethylnaphthalene isomers--that is, within the 1,5-, 1,6- and 2,6-dimethylnaphthalenes of triad A, within the 1,7-, 1,8-, and 2,7-dimethylnaphthalenes of triad B, and within the 1,3-, 1,4- and 2,3-dimethylnaphthalenes of triad C. It is also known that the interconversion of a dimethylnaphthalene isomer within one of the aforesaid triads to a dimethylnaphthalene isomer within another of the aforesaid triads occurs to a relatively lesser extent. However, it is highly desired to improve the selectivity and completeness of the aforesaid dehydrogenation and isomerization steps in the aforesaid multistep synthesis for the formation of the specific dimethylnaphthalene isomer(s) desired.